Definition

Thyroid peroxidase is a type of protein (called an enzyme) that cells in the thyroid gland use to make thyroid hormone. If these enzymes leak out into the bloodstream (as when thyroid cells are damaged) the immune system makes antibodies against these proteins. The thyroid peroxidase antibody test measures these antibodies in the blood.

Alternative Names

Thyroid antimicrosomal antibody; Antimicrosomal antibody; Microsomal antibody; Antithyroid microsomal antibody; TPOAb; Anti-TPO antibody

How the Test is Performed

A blood sample is needed.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise. This soon goes away.

Why the Test is Performed

This test is done to confirm the cause of thyroid problems, including Hashimoto thyroiditis.

The test is also used to find out if an immune or autoimmune disorder is damaging the thyroid.

Normal Results

A negative test means the result is normal.

Normal value ranges may vary slightly among different laboratories. Some labs use different measurements or may test different specimens. Talk to your health care provider about the meaning of your specific test results.

What Abnormal Results Mean

A positive test may be due to:

• Granulomatous or subacute thyroiditis (an immune reaction of the thyroid gland that often follows an upper respiratory infection)
• Hashimoto thyroiditis (a reaction of the immune system against the thyroid gland)

High levels of these antibodies have also been linked with an increased risk of:

• Miscarriage
• Preeclampsia (high blood pressure and protein in the urine after the 20th week of pregnancy)
• Premature birth
• In vitro fertilization failure

Important: A positive result does not always mean that you have a thyroid condition or that you need treatment for your thyroid. A positive result may mean that you have a higher chance of developing thyroid disease in the future. This is often associated with a family history of thyroid disease.

Antithyroid microsomal antibodies may be seen in your blood if you have other autoimmune conditions, including:

• Autoimmune hemolytic anemia
• Autoimmune hepatitis
• Autoimmune adrenal disease
• Rheumatoid arthritis
• Sjögren syndrome
• Systemic lupus erythematosus

Risks

There is little risk involved with having your blood taken. Veins and arteries vary in size from one person to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight, but may include:

• Excessive bleeding
• Fainting or feeling lightheaded
• Multiple punctures to locate veins
• Hematoma (blood buildup under the skin)
• Infection (a slight risk any time the skin is broken)

References

Brent GA, Weetman AP. Hypothyroidism and thyroiditis. In: Melmed S, Auchus RJ, Goldfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 13.

Guber HA, Oprea M, Russell YX. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Philadelphia, PA: Elsevier; 2022:chap 25.

Pearce EN, Hollenberg AN. Thyroid. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 207.

Ramos-Levi AM, Marazuela M. Thyroiditis. In: Robertson RP, ed. DeGroot's Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 73.

Smith JR, Wassner AJ. Thyroiditis. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 604.